Some old members may remember this but several years back we had a Rv.net member who I think had a tri axle TH ?? with the same problem. His rig was a handful with the garage loaded. He worked with his trailer mfg and the solution was for them to reposition the axles under the unit.
I've had this done by a big rig trailer repair shop on a tri axle GN stock trailer.
Another option is 8k axle upgrades if moving things around fails.
IMO NHTSA screwed the pooch with their GVWR rules change in early '00s. They issued a NPRM for CFR 571 rules change regarding setting a rv trailers GVWR.
Consumer groups were pushing rules changes to same as a commercial trailers/motor vehicles with the sum of the axle ratings as its GVWR. However they caved in to RVIA push to allow lower axle rating than the units GVWR. They saved a few bucks.
All my commercial class trailers GVWR has always been the sum of its axle rating.